1. Field of the Invention
The present invention relates to a wireless ad-hoc communication system. In particular, the invention relates to synchronization architecture for communication terminals in a wireless ad-hoc communication system.
2. Description of Related Art
Nowadays, in IEEE802.15 wireless personal area network (PAN) working group, extensive studies about media access control (MAC) of short-range wireless communications have been made.
There is no access point on the network of the wireless PAN unlike a wireless LAN (Local Area Network) complying with the IEEE802.11 series, so that data transmission/reception is directly executed between terminals as network elements. As remarked above, a feature of the wireless PAN resides in ad-hoc and peer-to-peer data transmission/reception between the terminals.
Another feature of the wireless PAN is to support synchronous data transfer (isochronous transfer) by means of TDMA (Time Division Multiplexing) as well as asynchronous data transfer (asynchronous transfer) based on access control such as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) despite the fact that there is no access point, at the time of transferring data between terminals.
For the TDMA-based isochronous transfer, it is necessary to synchronize terminals as network elements to match phases of time slots thereof. To synchronize the terminals, for example, in the IEEE802.15.3 standards, a device called Piconet controller (PNC) has been introduced. In the IEEE802.15.3 standards, PNC and communication terminals controlled by the PNC are collectively referred to as “Piconet”.
FIG. 9 shows the network structure conforming to the IEEE802.15.3 standards. A PNC 61 periodically transmits a beacon to synchronize communication terminals 62 to 64, and allocates time slots in a superframe following the beacon to the communication terminals 62 to 64. In this way, in the IEEE802.15.3 standards, network elements can be synchronized under the control of the PNC.
Incidentally, if a communication terminal belonging to a given Piconet is moved within a radio wave reachable range of another Piconet, synchronization between superframes of the Piconets is not achieved, resulting in interference of radio waves. For example, as shown in FIG. 10A, Piconet A and Piconet B approach each other and then, as shown in FIG. 10B, radio wave reachable ranges of the Piconets overlap with each other.
FIG. 11A shows superframes of the Piconets A and B that approach each other as shown in FIG. 10B. Reference numerals 601 to 603 denote beacons sent from the PNC 61, and 611 and 612 denote CTA time slots for isochronous transfer used in the Piconet A. Likewise, reference numerals 651 to 653 denote beacons sent from a PNC 65, and 661 to 665 denote CTA time slots for isochronous transfer used in the Piconet B. In FIG. 11A, transmission timings of the beacons 601 to 603 in the Piconet A correspond to the allocated CTA time slots 661, 663, and 665 of the Piconet B, respectively. In addition, CTA time slots 611 and 612 of the Piconet A overlap CTA time slots 662 and 664 of the Piconet B. As mentioned above, if interference occurs between beacons or time slots used for isochronous transfer, critical communication failures occur.
To that end, there has been proposed a method of preventing interferences among time slots used for the isochronous transfer in such a manner that the PNC that controls individual Piconets synchronize superframes and reallocates time slots to establish a cooperative relation (see Japanese Unexamined Patent Publication No. 2003-143644, for instance). As shown in FIG. 11B, the PNC 61 and the PNC 65 reallocate time slots not to overlap transmission/reception timings thereof.
Meanwhile, there have been hitherto made studies on a control method for synchronizing communication terminals necessary for TDMA-based isochronous transfer without introducing a control station such as the PNC. One of the MAC techniques to execute such control is MBOA MAC which is being under study in WiMedia-MBOA (Wimedia-Multiband-OFDM Alliance).
The MBOA MAC adopts a common beacon interval called a superframe for supporting both of the isochronous transfer and the asynchronous transfer. In this respect, the MBOA MAC is the same as the IEEE802.15.3 standards. Communication terminals adopting the same beacon interval are collectively referred to as a beacon group. The communication terminals of the beacon group execute isochronous transfer and asynchronous transfer with phase-coherent time slots in the superframe. Incidentally, since there is no control station corresponding to the PNC on the MBOA network, the communication terminals send individually beacons to achieve synchronization of the superframes.
Further, similar to the MBOA MAC, MAC for wireless ad-hoc communications without the PNC has been disclosed in S. Datta et al. “Ad-hoc extensions to the 802.15.3 MAC Protocol” at the Internet URL: http://paul.rutgers.edu/˜sdatta/wowmom.pdf (a search was made online for this specification on Apr. 14, 2005). The MAC disclosed in this publication defines a beacon period divided into plural beacon slots and set at the head of a superframe defined by beacon intervals. A communication terminal intended to participate in a working network listens beacons from existing communication terminals, and sends its own beacon at a free beacon slot. Through these operations, each communication terminal can use a common superframe to achieve synchronization necessary for isochronous transfer.
FIG. 12 shows a configuration example of a wireless ad-hoc communication system having no PNC and adopted in the above MBOA MAC. All communication terminals 71 to 74 individually send beacons and are autonomously synchronized, and eventually, the entire system can converge on a common beacon interval and superframe.
Incidentally, similar to the IEEE802.15.3 standards, in the MBOA MAC or the MAC disclosed in S. Datta et al.'s paper as well, radio wave interference occurs if beacon groups approach one another. In order to avoid the above interference, S. Datta et al.'s paper discloses such a technique that nodes belong to a given network adjust a starting position of a beacon period to a starting position of a beacon period of another network to synchronize each other's superframes.
FIGS. 13A to 14B illustrate how communication terminals operate when wireless ad-hoc communication networks having no PNC approach each other. FIG. 13A shows a state in which the network C is independent of the network D. That is, beacons sent from the communication terminals 81 to 83 constituting the network C do not reach communication terminals 84 and 85 constituting the network D, and vice versa (beacons sent from the communication terminals 84 and 85 do not reach the communication terminals 81 to 83).
FIG. 14B shows a superframe corresponding to the arrangement of the communication terminals of FIG. 13A. Beacons 811 to 813, 821 to 823, and 831 to 833 sent from communication terminals 81 to 83 constituting the network C are synchronized with one another and arranged in their superfame. In this way, the communication terminals 81 to 83 are synchronized with a common basic superframe length. On the other hand, beacons 841 to 843 and 851 to 853 sent from communication terminals 84 and 85 constituting the network D are synchronized with each other and arranged in their superfame. In this way, the communication terminals 84 and 85 are synchronized with a common basic superframe length. However, since the network C and the network D do not exchange beacons, starting positions of the superframes of the network C and the network D do not match, and time slots (e.g., CTS time slots 814 and 844) for isochronous transfer of the networks C and D overlap with each other on the time axis.
FIG. 13B shows how the networks C and D approach each other and converge on one synchronized network E. FIG. 14B shows a superframe in this case. FIG. 14B shows how to adjust a starting position of a beacon period of the communication terminals 84 and 85 belonging to the network D to a starting position of a beacon period of the network C to thereby synchronize superframes. After synchronizing the superframes, interfering time slots are reallocated.
As mentioned above, if the two networks approach, in the IEEE802.15.3 standards, it is necessary to establish a cooperative relation between the Piconets and achieve synchronization between superframes. Further, in the MBOA MAC, it is necessary to merge beacon groups to achieve synchronization between superframes. Also in the MAC disclosed in S. Datta et al.'s paper, it is necessary to adjust the starting positions of beacon periods to match phases of superframes, that is, adjust superframe lengths and superframe start timings. In such a process of reconstructing the network for synchronization, there is a high possibility of a interruption of communications between communication terminals.
In the wireless PAN, each communication terminal has high mobility. Thus, it is necessary to frequently execute synchronization process each time the communication terminal is moved. This results in a problem that network synchronization largely influences communications between communication terminals.
Especially in the MBOA MAC or the MAC disclosed in S. Datta et al's paper, even though some communication terminals of one beacon group interfere with another beacon group, all communication terminals of the one beacon group should match their phases of superframes with that of the other beacon group. Hence, an influence of the network synchronization on communications between the communication terminals is particularly large.